High spectral resolution of GaAs/AlAs phononic cavities by subharmonic resonant pump-probe excitation

Abstract

We present here precise measurement of the resonance frequency, lifetime and shape of confined acoustic modes in the tens of GHz regime in GaAs/AlAs superlattice planar and micropillar cavities at low temperature ($\sim 20\text{K}$). The subharmonic resonant pump-probe technique, where the repetition rate of the pump laser is tuned to a subharmonic of the cavity resonance to maximize the amplitude of the acoustic resonance, in combination with a Sagnac interferometer technique for high sensitivity ($\sim 10\text{fm}$) to the surface displacement, has been used. The cavity fundamental mode at $\sim 20\text{GHz}$ and the higher order cavity harmonics up to $\sim 180\text{GHz}$ have been clearly resolved. Mechanical Q-values up to $2.7\times 10^4$ have been measured in a planar superlattice, and direct spatial mapping of confined acoustic modes in a superlattice cavity micropillar has been demonstrated. The Q-frequency product obtained is $\sim 5\times 10^{14}$ demonstrating the suitability of these superlattice cavities for optomechanical applications.

5 pages, 4 figures